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<title>10.2 hmac -- Keyed-Hashing for Message Authentication</title>
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<h1><a name="SECTION0012200000000000000000">
10.2 <tt class="module">hmac</tt> --
         Keyed-Hashing for Message Authentication</a>
</h1>

<p>
<a name="module-hmac"></a>

<p>

<span class="versionnote">New in version 2.2.</span>

<p>
This module implements the HMAC algorithm as described by <a class="rfc" id='rfcref-100439' xml:id='rfcref-100439'
href="http://www.faqs.org/rfcs/rfc2104.html">RFC 2104</a>.

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2140' xml:id='l2h-2140' class="function">new</tt></b>(</nobr></td>
  <td><var>key</var><big>[</big><var>, msg</var><big>[</big><var>, digestmod</var><big>]</big><var></var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
  Return a new hmac object.  If <var>msg</var> is present, the method call
  <code>update(<var>msg</var>)</code> is made. <var>digestmod</var> is the digest
  constructor or module for the HMAC object to use. It defaults to 
  the <code><tt class="module"><a href="module-hashlib.html">hashlib</a></tt>.md5</code> constructor.  <span class="note"><b class="label">Note:</b>
The md5 hash
  has known weaknesses but remains the default for backwards compatibility.
  Choose a better one for your application.</span>
</dl>

<p>
An HMAC object has the following methods:

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2141' xml:id='l2h-2141' class="method">update</tt></b>(</nobr></td>
  <td><var>msg</var>)</td></tr></table></dt>
<dd>
  Update the hmac object with the string <var>msg</var>.  Repeated calls
  are equivalent to a single call with the concatenation of all the
  arguments: <code>m.update(a); m.update(b)</code> is equivalent to
  <code>m.update(a + b)</code>.
</dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2142' xml:id='l2h-2142' class="method">digest</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
  Return the digest of the strings passed to the <tt class="method">update()</tt>
  method so far.  This string will be the same length as the
  <var>digest_size</var> of the digest given to the constructor.  It
  may contain non-ASCII characters, including NUL bytes.
</dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2143' xml:id='l2h-2143' class="method">hexdigest</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
  Like <tt class="method">digest()</tt> except the digest is returned as a string
  twice the length containing
  only hexadecimal digits.  This may be used to exchange the value
  safely in email or other non-binary environments.
</dl>

<p>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2144' xml:id='l2h-2144' class="method">copy</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
  Return a copy (``clone'') of the hmac object.  This can be used to
  efficiently compute the digests of strings that share a common
  initial substring.
</dl>

<p>
<div class="seealso">
  <p class="heading">See Also:</p>

  <dl compact="compact" class="seemodule">
    <dt>Module <b><tt class="module"><a href="module-hashlib.html">hashlib</a></tt>:</b>
    <dd>The python module providing secure hash functions.
  </dl>
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